1. Field of the Invention
The present invention relates to a zinc oxide-filled addition-curable silicone rubber composition having an excellent storage stability and being practical, a zinc oxide-filled addition-curable silicone rubber composition having a high thermal-conductivity, and cured products thereof.
2. Description of the Prior Art
It is known that silicone rubber compositions are filled with zinc oxide in order to, for example, accelerate curing and improve the heat resistance and electrical insulation properties of cured products thereof. Conventionally, the filling of zinc oxide is carried out for millable silicone rubber compositions which are crosslinked and cured by organic peroxides, and room temperature-curable (RTV) silicone rubber compositions which are crosslinked and cured by condensation reaction, but scarcely carried out for addition-curable silicone rubber compositions which are crosslinked and cured by addition reaction. This is because on filling zinc oxide in the addition-curable silicone rubber composition, an organohydrogenpolysiloxane contained as a crosslinking agent in the composition is reacted with the basic zinc oxide to generate hydrogen gas and because the composition foams during storage due to the hydrogen gas thus generated the inside pressure of a closed vessel in which the composition is contained increases to cause a danger of breakage of the vessel.
However, the addition-curable silicone composition, unlike the condensation-curable RTV silicone rubber composition, has advantages that it does not require a long time for curing and that polysiloxane as a constituent component does not undergo cleavage, so-called reversion, under the condition of elevated temperatures in a closed system. Thus, there is required an improvement in properties by filling with zinc oxide with the advantages above being retained.
Further, since the cured products (silicone rubber elastomers) of the conventional addition-curable silicone rubber compositions have an insufficient adhesion to various metals and semiconductors, such as nickel, aluminum, silicon and copper (particularly, copper and copper oxides), an improvement in adhesion is also required.
While, as a high thermal-conductive silicone rubber composition for providing a cured product with a high thermal-conductivity as well as physical properties such as excellent mechanical properties, there have hitherto been proposed many compositions filled with a variety of high thermal-conductive fillers, such as an aluminum powder, a copper powder, a nickel powder and an alumina powder [e.g., Japanese Patent Publication (kokoku) No.5-36456 corresponding to U.S. Pat. No. 5,021,494 and Japanese Pre-examination Patent Publication (kokai) No.8-295737 corresponding to U.S. application Ser. No. 08/427535 (filed on Apr. 24, 1995, now abandoned) which further contains zinc oxide].
However, since generally these high thermal-conductive fillers have a high specific gravity and a large particle size, it is difficult to uniformly disperse the filler in the composition and the filler is liable to cause separation or sedimentation with the lapse of time, so that the composition is required to be again stirred (redispersed) prior to use (cure). Furthermore, the filling of the filler in a large amount results in the problem that adhesion to substrates, especially such as metals and semiconductors, is lowered. Thus, there is required a silicone rubber composition easy to deal with, which provides a cured product with a high thermal-conductivity without lowering adhesion even when used in a large amount.